Well it depends at what age you'd sell them at... Most people dont really want to pay TOO much for a rooster so i would sell them around 15-20$, Its also very important to mention that they're pure bred, Raised in great conditions, what type of feed you use (That one is optional) and their behavior, The more behaved/mature rooster you have the higher the price is
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Now for an auction i think you'd already know but you would mention a small price to start off with then if people decided they want them then the price will get higher and higher until they are sold...